Visitors and residents near Foster City's Gull and Marlin parks may have witnessed a natural phenomenon last weekend as thousands of anchovies died and washed ashore on the lagoon's beaches because their sheer volume depleted the oxygen they needed.

Once city officials noticed the dead fish washing ashore and in the lagoon, which is fed by the Bay, they immediately began to test the water and have since cleaned most of them up, said Public Works Superintendent Mike McElligottt. The quality of the water was safe, however, it did show it was depleted of more oxygen than usual, McElligottt said.

Although the event was unusual for Foster City and hasn't happened in at least the 10 years McElligottt said he's worked for the city, there is a biological explanation for it.

"This particular incident has not happened. But we have had fish die off about five or six years ago due to a red tide," McElligottt said. "We tested (the lagoon) for dissolved oxygen, it was low in those areas and I didn't realize what was going on until I called the National Marine Fisheries Service."

Sometimes anchovies school so close together or get stuck in a narrow water body that over a period of time they deplete the water of oxygen and consequentially suffocate, said Steve Miller, administrative officer for the Fisheries Ecology division in Santa Cruz, a sub-branch of the National Oceanic and Atmospheric Administration.

Incidents such as this are uncommon, but the Santa Cruz Harbor experienced a similar and larger anchovy event when millions died around July 31, Miller said. The harbor is narrow and increased sightings of whales and seagulls can be an indicator that anchovies are nearby, Miller said.

"The anchovies had been along the coast for several months and that's what brought the whales in and there's been huge swarms of seagulls, Miller said. "It probably took a week for them (anchovies) to accumulate, then they get trapped and then the water gets inhaled and goes through their gills. And that's what gills do, it filters oxygen out of the water so they basically suffocate."

McElligott said the city wasn't aware that there were specifically anchovies in the lagoon and it can be hard to tell.

"We don't do a check to see what kind of fish are in there, but since we let water in from the Bay, we don't know what sea creatures are getting in," McElligott said. "With the water the way it is, it's murky because it's brackish like the Bay. ... Also, we dye the lagoon a blue color and the reason for dying it is not just to have it look pretty, it's to prevent algae and subsurface grass from growing so we don't have to kill it with chemicals."

Since the anchovies appeared, the city has cleared them from the beaches and it's likely those that hadn't washed ashore have submerged, McElligott said. The city has also left labeled containers for residents to dispose of any they find on their property. The containers will be emptied twice daily, in the morning and afternoon in an effort to reduce odors. Plus, the seagulls have certainly helped out, McElligott said.

Miller said it's difficult to prevent a natural incident such as this from occurring, short of completely blocking off the water. Although anchovy die-offs such as Santa Cruz's and Foster City's are uncommon, they are showing up more than usual this year, Miller said.

"This has been a big year for anchovies and it has to do with the water temperature," Miller said. "It's been a warm year in the water in Central California and that's positive for anchovies. So we're kind of in a heavy anchovy cycle."

Anyone with questions about the Foster City anchovy die-off can call the city's Public Works Maintenance Division at (650) 286-8140.